Weird Tears in terrain mesh in Render View only
The problem is caused by the fact that although the terrain origin is at (0,0,0), the real mesh is still very far from it.

So it seems you have to use a very large clipping range (1000000 in your case), which allows you to see objects, but reduces the depth precision. This is a known limitation of all softwares using OpenGL.

To fix it, you can:


Increase the start clipping.
Decrease the end clipping.


However, you may also have to scale the objects to see it again. (And, in your case, I would suggest you select the terrain then ShiftCtrlAltC > Origin to Geometry, to relocate the origin from afar. Then move them back to the world center.)